Method for the non-destructive testing of a casing by colorimetry

ABSTRACT

A method for non-destructively testing the heating of a determined zone of a part made of a polymer material, the method comprising the following steps: a) taking at least one colorimetric measurement on said determined zone to be tested and obtaining the value ap of the parameter a of the CIELAB colorimetric space; b) taking at least one colorimetric measurement on said reference zone of said part and obtaining the value ap/ref of the parameter a of the CIELAB colorimetric space; c) calculating Δap=ap−ap/ref; and d) establishing a risk of heating said determined zone to be tested if Δap is higher than a threshold value A1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent No. 62/462,154, filed Feb. 22, 2017, and French Patent Application No. 1751896, filed Mar. 8, 2017, the contents of each of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method for non-destructively testing a part comprising a matrix, for example, made of polymer. In particular, reinforcing fibres can be integrated to the matrix.

BACKGROUND

Conventionally, the upstream end of a turbine engine comprises a fan comprising a wheel formed of a plurality of blades surrounded on the outside by an annular casing which can made of a metal material or of a composite material comprising a matrix integrating reinforcing fibres, such as a polymer matrix, for example, epoxide polymer, and carbon fibre or glass fibre reinforcing fibres. This casing enables an initial compression of air entering in the turbine engine and also ensures a function for confining the blades in case of loss of one of them. The fan casing is surrounded by a plurality of equipment supply ducts, in particular, by a pressurised air supply duct, at a temperature of around 200° C., an engine called APU (auxiliary power unit) used to start the turbojet as well as supplying the aircraft cabin with electricity when the aircraft is grounded.

In case of malfunctioning, such as a leak from the pressurised air supply duct, the air can lead to a significant local heating of the casing, since the temperature of the air is around 200° C. When the casing is made of a metal material, for example, aluminium, the heating is not impacted by the mechanical integrity of the casing. In the case of a matrix fan case integrating the reinforcing fibres, the mechanical strength thereof following being heated, must be able to be guaranteed.

Therefore, it is understood that the non-destructive test of a composite matrix casing with reinforcing fibres is particularly important, and all the more so, as a composite casing is proving to be very expensive.

It has thus been proposed to apply thermosensitive paints on the casing. However, the lifespan of these paints highly limits their interest, since an engine can be used for period longer than the lifespan of these paints, in particular for long- or medium-haul aircraft. Furthermore, during the placing of an engine, this conventionally undergoes cleaning by scraping, which causes a total removal of the layer of thermosensitive paint, involving another step of applying a layer of paint. Finally, if a thermosensitive paint enables to visually consider the heating state of a given zone of a fan case, it only proves to be an indirect measurement of the state of the internal structure of the casing and does not enable a specific quantification of the internal structure of the casing.

SUMMARY

The invention, in particular, aims to provide a simple, effective and economic solution to the problems of the prior art defined above.

To this end, it proposes a method for non-destructively testing the heating of a zone determined to be tested of a part made of a polymer material, the method comprising the following steps:

-   -   a) taking at least one colorimetric measurement on said         determined zone to be tested and obtaining the value a_(p) of         the parameter a of the CIELAB colorimetric space,     -   b) taking at least one colorimetric measurement on said         reference zone of the part and obtaining the value a_(p/ref) of         the parameter a of the CIELAB colorimetric space,     -   c) calculating Δa_(p)=a_(p)−a_(p/ref),     -   d) establishing a risk of heating said determined zone to be         tested if Δa_(p) is higher than a threshold value A1.

The method according to the invention enables to determine if the examined part has been subjected (or not) to an overheating beyond an acceptable limit. In practice, if a risk of overheating is established, a physicochemical analysis of the part is carried out so as to determine, more specifically, the level of overheating. However, a physicochemical analysis must be carried out in a laboratory, which requires a disassembly of the part from the mechanical unit whereon it is assembled. Thus, when the part to be tested is a turbine engine casing, the laboratory test involves a placing of the aircraft engine and highly increases the immobilisation time of the aircraft and consequently the operating costs.

The invention proposes to establish a risk of heating a given zone of a part, compared with a supposedly normal reference section, in other words, a section that has not been subjected to heating, of the examined part, which enables to establish a reference on the part itself, in order to consider the normal variations of the parameter a due to the outside environment wherein the part has been since it was manufactured. The term “normal” refers to the standard conditions of use of the part, enabling to have a predetermined lifespan of the part.

In the CIELAB colorimetric space, the variation in the value of the parameter a gives information on the level of overheating subjected to by the determined zone to be tested of the part.

The threshold A1 can be determined by using a reference database comprising colorimetric measurements taken on a plurality of reference samples made of a polymer material, in particular, made of reinforcing fibres, having been subjected to a determined temperature for a predetermined time period. From a plurality of measurements of the parameter a, a threshold A1 is established, below which the mechanical integrity of the part cannot be guaranteed without a physicochemical analysis.

The value Δa_(p) gives information on the (component) level of red colour present in the given zone which is tested.

In the application, the CIELAB colorimetric space means the CIE L*a*b* system or the acronym CIE means the Commission Internationale de l'Eclairage (International Lighting Commission). The asterixis have voluntarily been omitted in the text to avoid encumbering the notations.

In the present document, the CIELAB space corresponds to that defined by the standard NF EN ISO 11664-4 (2011-07-01), of which the title is “Colorimétrie—Partie 4: espace chromatique L*a*b* CIE 1976” (“Colorimetry—Part 4: CIE L*a*b* colour space 1976”).

The method can further comprise the following steps:

-   -   obtaining the value b_(p) of the parameter b and the value L_(p)         of the parameter L of said at least one colorimetric measurement         taken in step a),     -   obtaining the value b_(p/ref) of the parameter b and the value         L_(p/ref) of the parameter L of said at least one colorimetric         measurement taken in step b),     -   calculating Δb_(p)=b_(p)−b_(p/ref) and calculating         ΔL_(p)=L_(p)−L_(p/ref),     -   establishing a risk of heating said determined zone to be         tested, if all the following conditions are checked:         -   Δa_(p) is higher than a threshold value A2, A2 being lower             than A1,         -   Δb_(p) is higher than a threshold value B1,         -   ΔL_(p) is lower than a threshold value L1.

The value Δb_(p) gives information on the level of yellowing of the determined zone which is tested. It is reminded that, in this colour space, the value of the parameter b develops from negative values up to positive values, in other words, from blue to yellow. The variation in value of the parameter L gives information on the development of the luminance/clarity and develops from the value 0, corresponding to black, up to the value 100, corresponding to white. The threshold L1 enables to give information on the decrease in clarity of the tested zone, in relation to the reference zone.

Thus, if during step d), the risk of overheating is not established, the method consists of making three successive comparisons, of which the positive results imply the establishment of a risk.

According to a characteristic of the invention, the thresholds A2, B1, L1 are determined by using said colorimetric measurements stored in the reference database.

According to another characteristic of the invention, establishing the database comprises the following steps:

-   -   for each first sample, obtaining the value a′ of the parameter a         of the CIELAB colorimetric space, from at least one colorimetric         measurement,     -   for each first sample, calculating Δa′=a′−a′_(ref), where:         -   α′_(ref) corresponds to the value of the parameter a of the             CIELAB colorimetric space, this value having been obtained             on a second sample made of a polymer material, in             particular, made of reinforcing fibres, having the same time             of existence as said first sample considered and having been             kept at a temperature within a range of temperatures, such             as that of preserving the mechanical integrity of the second             sample, or such as a range between 0 and 40° C., which could             further consider the exposure to rays in the ultraviolet             field,     -   carrying out a test to determine the mechanical properties of         each one of the first samples,     -   determining the threshold A1 from comparing the data from the         tests carried out in the previous step and the values Δa′         contained in the database.

Establishing the database also comprises the following steps:

-   -   for each first sample, obtaining the values L′ and b′ of the         respective parameters L and b of the CIELAB colorimetric space,         from at least one colorimetric measurement,     -   for each first sample, calculating Δb′=b′−b′_(ref) and         ΔL′=L′−L′_(ref), where:         -   b′_(ref) and L′_(ref) respectively correspond to the values             of the parameters b and L of the CIELAB colorimetric space,             these values having been obtained on a second sample made of             a polymer material, in particular, made of reinforcing             fibres, having the same time of existence as said first             sample considered and having been kept at a temperature             within a range of temperatures, such as that of preserving             the mechanical integrity of the second sample, or such as a             range between 0 and 40° C., which could further consider the             exposure to rays in the ultraviolet field,     -   determining the thresholds A2, B1 and L1 from comparing the data         from said tests and the values Δa′, Δb′ and ΔL′ contained in the         database.

According to the invention, testing the mechanical properties of the first samples can comprise at least one step for mechanically testing the part, for example, by traction and/or compression.

The method can again comprise the following steps:

-   -   calculating the colour difference ΔE_(p) between the         colorimetric measurements obtained in steps a) and b),     -   from the value ΔE_(p) obtained in the previous step, determining         the time period during which said zone to be tested of the part         has been subjected to a determined heating temperature and         determining said heating temperature, by using a reference         database comprising the values ΔE′ of colour differences         obtained from a plurality of samples made of a polymer material         having been subjected to a determined temperature for a         determined time period.

The value ΔE_(p) is determined for said zone to be tested by making the calculation ΔE_(p)=√{square root over (ΔL_(p) ²+Δa_(p) ²+Δb_(p) ²)}.

The values ΔE′ are determined for each first sample by making the calculation ΔE′=√{square root over (ΔL′²+Δa′²+Δb′²)}.

To limit errors in measurement and to average out the experimental variability, each value considered of the parameters L, a, b of the CIELAB colorimetric space can be obtained by taking the average of at least five successive colorimetric measurements at the place considered.

According to the invention, in case a heating risk is established, the method can also further comprise a subsequent step to step d), of carrying out a physicochemical analysis of the determined zone to be tested of the part, so as to determine on the state of damage of said determined zone.

The part analysed can be made of a polymer material comprising reinforcing fibres.

Preferably, a step of cleaning the surface whereon a colorimetric measurement is intended to be taken, is carried out prior to said colorimetric measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be best understood, and other details, advantages and characteristics of the invention will appear upon reading the following description, made as a non-exhaustive example, in reference to the following figures:

FIG. 1 is a schematic view of a turbine engine fan case to be tested;

FIG. 2 is a representative view of the CIELAB colorimetric space axes used in the present document;

FIG. 3 is a schematic view of a plurality of samples, each having undergone oxidation for a given time (vertically) at a given temperature (horizontally);

FIG. 4 is a schematic view illustrating a line of separation between a first zone 1 and a second zone 2;

FIG. 5 is a schematic view of a graph where each point represents a sample from FIG. 3 having been subjected to a vitreous transition temperature of the tested material, each point being placed on the graph according to the values of the parameters a and b, thereof, a being represented on the x-axis and b being represented on the y-axis;

FIG. 6 is a schematic view of a graph where each point represents a sample from FIG. 3 having been subjected to a higher temperature than the vitreous transition temperature of the polymer material, each point being placed on the graph according to the values of the parameters a and b, thereof, a being represented on the x-axis and b being represented on the y-axis;

FIG. 7 is a flowchart of the functioning of the decision-making method during a step of non-destructive testing on a given zone to be tested, of a determined part such as the casing from FIG. 1;

FIG. 8 is a graph representing the development in the colour difference over time for the samples from FIG. 3;

FIG. 9 is a larger-scale graph of a given zone of the graph from FIG. 8.

DETAILED DESCRIPTION

As explained above, the fan casing 10 represented in FIG. 1, made of polymer, in particular of reinforcing fibres, can undergo when switched on, a local heating that should be able to be characterised by a non-destructive method enabling to determine the state of the casing 10 in order to determine if it can (or not) be kept in service in a turbine engine.

FIG. 2 represents the CIELAB space used in the present document for analysing colorimetric data obtained on the part to be tested, as well as for establishing the reference database on a plurality of first samples. The CIELAB space is a system which enables to represent the trichromatic components along three orthogonal axes between them. The axis L (or L*) represents the luminance or clarity axis (perfect black: L=0; perfect white: L=100). The axis a (or a*) represents the axis going from green (negative values of a) to red (positive values of a). The axis b (or b*) represents the axis going from blue (negative values of b) to yellow (positive values of b).

It is again reminded, that the space used is the CIE L*a*b*colorimetric space, and that the asterixis have voluntarily been removed as this is usual.

In this colour system, a colour difference between a first colour of coordinates L₁,a₁, b₁ and a second colour of coordinates L₂, a₂, b₂ is calculated as follows:

${{\Delta \; E} = \sqrt{{\Delta \; L^{2}} + {\Delta \; a^{2}} + {\Delta \; b^{2}}}},{where}$ Δ L = (L₁ − L₂) Δ a = (a₁ − a₂) Δ b = (b₁ − b₂)

This method of calculating the colour difference is the one used later, as this will appear following the description.

The invention proposes to establish a reference database comprising colorimetric measurements according to the CIELAB colour system. The term “reference” used below is to be understood as meaning an element of the reference database comprising colorimetric measurements and more generally, data obtained from the reference samples.

For this, a batch of a plurality of first samples 12 of a material similar to the part to be analysed is constituted. FIG. 3 illustrates such a batch which thus comprises several first samples 12 of fan case 10 made of a polymer material, preferably of reinforcing fibres, arranged in the form of lines and columns. Along a given line, each first sample 12 is subjected to a given temperature, of which the exposure time is given by the position along a line. Of course, the database should comprise a number of first samples 12 enabling to realise different levels of thermal oxidation, that the polymer can be subjected to under actual functioning conditions. Thus, the database should comprise the first samples 12 having been subjected to the aforementioned temperatures for periods of time going up to at least 6 months.

In the configuration represented as an example, the first samples 12 have been subjected to temperatures in ° C. of 120, 140, 150, 160, 180, 200, 220 and 240° C. for periods in hours, spread out from 1 up to 1440 hours, which corresponds to a period of 2 months. The sample 14 positioned in the lower left-hand corner in FIG. 1 represents a sample 12 not having been subjected to any heating, which therefore constitutes the absolute reference of a fan case 10 without any thermal heating. It is observed in FIG. 1, that the first samples 12 darken as soon as the temperature increases, and that the exposure time to a temperature increases, which is consistent with a thermal oxidation of the polymer.

FIG. 4 schematically represents FIG. 3 and comprises a line of separation 16 of a first zone 1 and a second zone 2. The first zone 1 corresponds to the first samples 12 which have been subjected to an acceptable temperature for an acceptable time period, whereas the second zone 2 corresponds to the first samples 12 which have been subjected to too-high temperature for a given time period. Thus, if visually, it is possible to establish this line of separation, it seems necessary to establish one or several parameters enabling to objectively realise the state of an analysed part. That is what is defined below by establishing the reference database.

Establishing the reference database first consists of taking a colorimetric measurement for each one of the first samples and by deducing the values L′, a′ and b′ of the parameters L, a, b of the CIELAB colorimetric space, from at least one colorimetric measurement.

It must first be noted, that the values of the parameters L, a, b can be obtained from several colorimetric measurements in each zone where the measurement is taken, in other words, to establish the database or when it is wanted to test a determined zone of a part, as this will be explained later.

Now reference is made to FIGS. 5 and 6, each representing a graph where each point represents a first sample from FIG. 3, each point being placed on the graph according to the values of the parameters a and b thereof, a being represented on the x-axis and being represented on the y-axis. In FIG. 5, the first samples have been subjected to a temperature lower than the vitreous transition temperature of the polymer material, here 170° C., and in FIG. 6, the first samples have been subjected to a temperature higher than said vitreous transition temperature.

In FIG. 5, it has been observed that a group 18 of points is found at the values of the parameter having a value of less than zero whereas, in FIG. 6, for the first samples having been subjected to a temperature higher than 170° C., it has been observed that a group 20 of values of the parameter a are higher than zero. Consequently, it is possible to discriminate on the thermal state, in other words, the heating of a given zone of a part, from the measurement of this parameter. It will be noted that in FIG. 6, a second group 22 has values of the parameter a which are less than zero, but these points correspond to very low exposure times, less than 10 hours, which are not to be considered. On the graph in FIG. 5, it has also been observed that the variation is made mainly along the axis b, this variation enabling to highlight the yellowing by natural ageing over time of the polymer and of the resin in the case of a fan casing. This variation along the axis b is also visible in FIG. 6.

Thus, it is understood that it is possible, with a colorimetric measurement in the CIELAB space, to differentiate between the natural ageing of the fan case and an accidental overheating of by comparing with a reference database.

To each first sample 12, it is calculated that Δa′=a′−a′_(ref), Δb′=b′−b′_(ref) and ΔL′=L′−L′_(ref), where:

-   -   a′_(ref), b′_(ref) et L′_(ref) respectively correspond to the         values of the parameters a, b, L of the CIELAB colorimetric         space, these values having been obtained on a second sample 14         made of a polymer material, in particular, of reinforcing         fibres, having the same time of existence as said first sample         considered, and having been kept at a temperature within a range         of temperatures, such as that of preserving the mechanical         integrity of the second sample 14, or such as a range between 0         and 40° C., which could further consider the exposure to rays in         the ultraviolet field.

The colorimetric measurement on the second sample 14 associated with each measurement of a first sample 12 can be taken with the reference sample 14 that has been observed under the conditions stated in the previous paragraph.

For each first sample 12, a test is then carried out, aiming to determine the mechanical properties thereof, in order to determine the ability or not thereof to constitute a sample that can be used under the determined conditions. Thus, it is determined if the heating subjected by each first sample makes it useable or not. The test carried out can comprise at least one step for mechanically testing the part, for example, by traction and/or compression.

Finally, a comparison of the data from the tests with the values Δa′, Δb′ et ΔL′ contained in the reference database is made, in order to establish the thresholds A1, A2, B1 and L1. The threshold A1 corresponds to a threshold beyond which it is considered that the fan case 10 must be arranged to be subjected to a more in-depth inspection of the zone to be tested (FIG. 7).

To carry out a non-destructive test operation on the casing 10 from FIG. 1, first, the following steps are carried out, schematised in FIG. 7:

-   -   a) taking at least one colorimetric measurement on said         determined zone 24 to be tested of the casing 10 and obtaining         the value a_(p) of the parameter a of the CIELAB colorimetric         space,     -   b) taking at least one colorimetric measurement on a reference         zone 26 of the casing 10 and obtaining the value a_(p/ref) of         the parameter a of the CIELAB colorimetric space,     -   c) calculating Δa_(p)=a_(p)−a_(p/ref),     -   d) establishing a risk of heating said determined zone to be         tested if Δa_(p) is higher than a threshold value A1.

The reference zone 26 of the casing 10 is a zone which has not suffered thermal damage.

In the case where the value Δa_(p) is lower, a second step is carried out, aiming to determine if the part must undergo (or not) a test all the same, this time considering the value b_(p) of the parameter b and the value L_(p) of the parameter L obtained from the colorimetric measurement on the zone 24 to be tested of the casing, as well as the value b_(p/ref) of the parameter b and the value L_(p/ref) of the parameter L obtained from the colorimetric measurement on the reference zone 26 of the casing.

The method then consists of calculating Δb_(p)=b_(p)−b_(p/ref) and calculating ΔL_(p)=L_(p)−L_(p/ref), and establishing a risk of heating said determined zone 24 to be tested if all the following conditions are checked:

-   -   Δa_(p) is higher than a threshold value A2, A2 being lower than         A1,     -   Δb_(p) is higher than a threshold value B1,     -   ΔL_(p) is lower than a threshold value L1.

In this case, it is looked to determine if the tested zone 24 has a greater yellowing than the threshold value B1, if the clarity ΔL_(p) is low, in other words, lower than the threshold L1 while having a Δa_(p) higher than a threshold value A2 lower than A1.

If one of the conditions above is not checked, the zone 24 to be tested is considered as not being damaged and the casing 10 can be used.

The non-destructive testing by colorimetry operation can be carried out under the wing of the aircraft, which enables to have a quick and reliable decision regarding the placing or not of the casing and reduces the unnecessary maintenance operations.

The parameters A1, A2, B1, L1 must be established to each type of part 10 and are therefore connected to the material of said part and also depend on the colorimetric measuring device.

Thus, in an example of taking colorimetric measurements with a Konica Minolta CM700d colorimeter, A1 is equal to 4.3, A2 is equal to −1, B1 is equal to 12.6 and L1 is equal to −0.9.

From the aforementioned colorimetric measurements contained in databases, it is possible to determine the time period during which the tested zone has been subjected to determined temperature, as well as this temperature.

For this, the colour difference ΔE_(p)=√{square root over (ΔL_(p) ²+Δa_(p) ²+Δb_(p) ²)} is calculated for the tested zone by using the colorimetric measurements obtained on the tested zone and on the reference zone of the analysed casing. For each one of the first samples, the colour difference ΔE′=√{square root over (ΔL′²+Δa′²+Δb′²)} is also calculated from the values ΔL′, Δa′ and Δb′. From the values ΔE′, it is possible to trace the development of ΔE′ over time for several determined temperatures as this is represented in FIGS. 8 and 9.

FIG. 8 represents the development of ΔE′ over time for the temperatures 120, 140, 150 and 160° C., these curves are respectively referenced ΔE′₁₂₀, ΔE′₁₄₀, ΔE′₁₅₀ and ΔE′₁₆₀.

To avoid the colour difference development curves being impacted by the non-relevant values of ΔE′ because of a very clear colouring or a colouring that is too dark, the points associated with such values ΔE′ are removed. A very clear colouring can be due to an exposure to a low temperature for a relatively short time period. This zone corresponds to the zone 28 in FIG. 3 and cannot validly be considered in the colorimetric analysis. A too dark colouring can be due to an exposure to a too high temperature for a significant time period. This zone corresponds to the zone 30 in FIG. 3 and cannot validly be considered in the colorimetric analysis.

Therefore, the situation is limited to the relatively long time of exposure, longer than 300 hours, as this is represented in FIG. 9. With this graph, it is possible from a measurement ΔE_(p) obtained on a given zone of a casing, to determine the temperature of exposure by tracing the constant ordinate line ΔE_(p) and searching for the intercepted curve which therefore indicates the temperature subjected to by said tested zone and the exposure temperature represented by the abscissa of the intersection point. 

1. A method for non-destructively testing a heating of a determined zone to be tested of a part made of a polymer material, the method comprising the following steps: a) taking at least one colorimetric measurement on said determined zone to be tested and obtaining a value a_(p) of a parameter a of a CIELAB colorimetric space, b) taking at least one colorimetric measurement on a reference zone of said part and obtaining a value a_(p/ref) of the parameter a of the CIELAB colorimetric space, c) calculating Δa_(p)=a_(p)−a_(p/ref), d) establishing a risk of heating said determined zone to be tested if Δa_(p) is higher than a threshold value A1.
 2. The method according to claim 1, further comprising the following steps: obtaining a value b_(p) of a parameter b and a value L_(p) of a parameter L of said at least one colorimetric measurement taken in step a), obtaining a value b_(p/ref) of the parameter b and a value L_(p/ref) of the parameter L of said at least one colorimetric measurement taken in step b), calculating Δb_(p)=b_(p)−b_(p/ref) and calculating ΔL_(p)=L_(p)−L_(p/ref), establishing a risk of heating said determined zone to be tested, if all following conditions are checked: Δa_(p) is higher than a threshold value A2, A2 being lower than A1, Δb_(p) is higher than a threshold value B1, ΔL_(p) is lower than a threshold value L1.
 3. The method according to claim 2, wherein the threshold A1 is determined by using a reference database comprising colorimetric measurements taken on a plurality of first samples made of a polymer material, in particular, made of reinforcing fibres, having been subjected to a determined temperature for a determined time period.
 4. The method according to claim 3, wherein the thresholds A2, B1, L1 are determined by using said colorimetric measurements stored in the reference database.
 5. The method according to claim 4, wherein establishing the database comprises the following steps: for each first sample, obtaining a value a′ of the parameter a of the CIELAB colorimetric space, from at least one colorimetric measurement, for each first sample, calculating Δa′=a′−a′_(ref), where: a′_(ref) corresponds to a value of the parameter a of the CIELAB colorimetric space, the value having been obtained on a second sample made of reinforcing fibres, having a same time of existence as said first sample considered and having been kept at a temperature within a range of temperatures to preserve a mechanical integrity of the second sample, carrying out a test to determine mechanical properties of each one of the first samples, determining the threshold A1 from comparing data from the tests carried out in previous steps and the values Δa′ contained in the database.
 6. The method according to claim 5, wherein establishing the database also comprises the following steps: for each first sample, obtaining values L′ and b′ of the parameters L and b of the CIELAB colorimetric space, from at least one colorimetric measurement, for each first sample, calculating Δb′=b′−b′_(ref) and ΔL′=L′−L′_(ref), where: b′_(ref) and L′_(ref) respectively correspond to values of the parameters b and L of the CIELAB colorimetric space, these values having been obtained on a second sample made of reinforcing fibres, having a same time of existence as said first sample considered and having been kept at a temperature within a range of temperatures to preserve a mechanical integrity of the second sample, determining the thresholds A2, B1 and L1 from comparing data from said tests and the values Δa′, Δb′ and ΔL′ contained in the database.
 7. The method according to claim 5, wherein said test comprises at least one step for mechanically testing the part.
 8. The method according to claim 1, further comprising the following steps: calculating a color difference value ΔE_(p) between the colorimetric measurements obtained in steps a) and b), from the value ΔE_(p) obtained in the previous step, determining a time period during which said zone to be tested of the part has been subjected to a determined heating temperature and determining said heating temperature, by using a reference database comprising values ΔE′ of color differences obtained from a plurality of samples made of a polymer material having been subjected to a determined temperature for a determined time period.
 9. The method according to claim 2, wherein each value considered of the parameters L, a, b of the CIELAB colorimetric space is obtained by taking an average of at least five successive colorimetric measurements at a place considered.
 10. The method according to claim 1, wherein in case of establishing a risk of heating, the method further comprises a subsequent step to step d) for carrying out a physicochemical analysis of the determined zone to be tested of the part so as to determine a state of thermal damage of said determined zone to be tested.
 11. The method according to claim 1, wherein the polymer material comprises reinforcing fibres.
 12. The method according to claim 1, further comprising a step of cleaning a surface whereon a colorimetric measurement is intended to be taken.
 13. The method according to claim 5, wherein the range of temperatures is a range between 0 and 40° C.
 14. The method according to claim 6, wherein the range of temperatures is a range between 0 and 40° C.
 15. The method according to claim 7, wherein the at least one step for mechanically testing the part comprises at least one of traction and compression. 